This invention relates to the high voltage processing of color cathode ray tubes (CRTs), and more particularly relates to the reconditioning of the lower gap of mini-neck color CRTs.
In the manufacture of CRTs for color television, it is necessary to process the electron gun assembly (also called the "mount") after it has been sealed into the neck of the CRT, in order to minimize the occurrence of internal arcs during later CRT operation. Modern color CRTs are particularly susceptible to such internal arcing due to their relatively high operating potentials (e.g., 25 KV and higher), and complex electron gun structures having relatively small interelectrodes spacings (e.g., mils).
In high voltage processing (also called "conditioning" or "spot" knocking), internal arcing between electrodes is purposely induced to remove microscopic sources of field emission such as foreign particles and interelectrode projections, which could otherwise lead to detrimental arcing during later tube operation.
To be effective, such conditioning should induce arcing not only in the upper gap (gap between the final focusing electrode and the final accelerating electrode), but also in the lower gap (gap between the focusing electrode and the final grid electrode).
Various arrangements have been proposed for the high voltage conditioning of color CRTs. See, for example, U.S. Pat. Nos. 3,736,038; 4,052,776; 4,214,798; and 4,326,762.
Recently, several CRT manufacturers have begun to produce tubes having smaller diameter necks, so-called mini neck tubes. These tubes offer cost saving advantages to the TV set manufacturer and end user, such as smaller electron beam deflection yokes and lower power consumption.
Of course, the smaller size of the mini neck (22 mm diameter versus 29 mm diameter for the older narrow neck types) requires a smaller mount. Unfortunately, the smaller size and consequent smaller interelectrode spacings of the mini neck mounts as compared to the older narrow neck mounts leads to a greater susceptibility to arcing. Thus, the need for effective high voltage conditioning is especially critical for these mini neck mounts.
An arrangement which has been found to be particularly effective for both upper and lower gap conditioning of the newer mini neck types of CRTs is described and claimed in U.S. patent application Ser. No. 713,939, filed Mar. 20, 1985.
Despite the effective conditioning which each CRT undergoes, a small number of tubes still exhibit an unstable condition during a procedure called set-up, which is performed by the customer. One part of the set-up procedure is the setting of cut-off voltages on the mount components. Cut-off is simply the voltage value at which electron emission becomes zero. This condition is approached when the screen raster collapses to a horizontal line, and is indicated when the line disappears from the screen.
The unstable condition which occurs during setting of the cut-off voltages is a flickering of the horizontal line on the screen, making it difficult to determine the exact point of cut-off. This flickering is caused by a low level field emission from the G2 grid electrode, probably from the edges or surface of the electrode, at low work function sites. These sites are thought to be comprised of surface contaminants or irregularities such as projections.
It is an object of the invention to provide a method for eliminating this field emission from the G2 grid electrode.
It is another object of the invention to provide such a method which is simple and does not require the set-up and use of elaborate equipment.